The present invention relates to a method and apparatus for depositing a semiconductor film on a wafer by making source gases supplied flow almost horizontally to the surface of the wafer. The present invention also relates to a method for fabricating a semiconductor device by using the film deposition method or apparatus.
Group II-VI or III-V compound semiconductors are direct transition type semiconductors with wide bandgap energy, and are hopefully applicable to emitting light at various wave-lengths that range from visible through ultraviolet regions of the spectrum.
Among other things, Group III-V nitride semiconductors, including gallium (Ga) or aluminum (Al) as a Group III constituent and nitrogen (N) as a Group V constituent, have attracted much attention, because those semiconductors exhibit crystallographically excellent properties. Thus, a method for depositing a film of a nitride semiconductor just as intended is in high demand.
A metalorganic chemical vapor deposition (MOCVD) process has been researched and developed widely and vigorously as one of industrially implementable methods of promise.
Hereinafter, a so-called xe2x80x9chorizontal MOCVD reactorxe2x80x9d, which is so constructed as to make source gases flow horizontally to the wafer surface, will be described as a known semiconductor film deposition apparatus with reference to FIGS. 7A and 7B.
As shown in FIGS. 7A and 7B, the horizontal reactor 200 includes: reactor body 201; gas inlet tube 202 with a gas inlet port 221; and susceptor 211 attached to the bottom of the reactor body 201. In this case, the reactor body 201 and gas inlet tube 202 are made of quartz glass, for example. Also, a gas outlet port 212 is provided at the other end of the reactor body 201 on the opposite side to the gas inlet tube 202.
The susceptor 211 holds a wafer 100 thereon to heat the wafer 100 up to a predetermined temperature.
A source gas 101, supplied through the gas inlet port 221, should be a laminar flow with no vortices after the gas 101 enters the tube 202 through the inlet port 221 and until the gas 101 reaches the space over the susceptor 211. The gas 101 also needs to flow in such a manner as to show spatially uniform velocity distribution over the wafer 100 to grow compound semiconductor crystals of quality.
However, the opening width of the gas inlet port 221 is relatively small as defined by its manufacturing standard, and the gas, supplied through the inlet port 221, should expand to cover an area equal to or greater in width than that of the susceptor 211. For that purpose, the gas inlet tube 202 has an expanded portion 222, the width of which gradually increases from the gas inlet port 221 toward the susceptor 211. In this case, if the angle xcex1 of expansion of the expanded portion 222 is large, then a streamline, which has flowed along the inner wall surface of the tube 202, separates from the surface in a velocity boundary layer near the wall of the expanded portion 222 as shown in FIG. 7A. Then, the streamline flows backward, i.e., toward the gas inlet port 221, to turn into a separated streamline (or vortex stream-line) 102. Also, a wake, or a vortex 103, is created inside a curvature formed by the separated streamline 102. In other words, a backward flow, moving upstream along the wall surface of the expanded portion 222, is created and then separated from the wall surface at a separation point to form the separated streamline 102. In FIG. 7A, only the streamlines flowing along the wall on the left-hand side of the gas flow are illustrated. Actually, though, similar streamlines also flow along the right-hand-side wall surface almost symmetrically to the illustrated ones about the centerline.
If the vortex 103 is created in the expanded portion 222, then the channel width of the gas flow is substantially decreased or deformed. As a result, the velocity distribution of the gas flow over the susceptor 211 cannot be spatially uniform anymore. In addition, the source gas 101 gets partially stuck inside the vortex 103, thus adversely delaying the exchange of one source gas for another. In that case, even if the semiconductor film being deposited should have its composition changed, the interfacial profile cannot be steep enough.
To solve these problems, G. B. Stringfellow proposed expanding the sidewalls of the expanded portion 222 gently by setting the expansion angle xcex1 to 7 degrees or less (see xe2x80x9cOrganometallic Vapor-Phase Epitaxyxe2x80x9d, Second Edition, p. 364, Academic Press).
Another solution is disposing a netlike or porous diffuser 223 in the expanded portion 222 of the gas inlet tube 202 as shown in FIGS. 8A and 8B or 9A and 9B to prevent the vortex from being created in the expanded portion 222.
However, the known horizontal reactor 200 has the following drawbacks. Specifically, if the expansion angle xcex1 of the expanded portion 222 is set to about 7 degrees or less, then the distance from the gas inlet port 221 to the gas outlet port 212 of that reactor 200 becomes very long. Accordingly, it may take an excessively large area to dispose such a bulky reactor. Or that long reactor may break very easily, so too much care should be taken in handling such a reactor.
On the other hand, if the diffuser 223 is disposed inside the gas inlet tube 202, then the spatial uniformity in the velocity distribution of the gas flow improves. Nevertheless, the gas flow is reflected by the diffuser 223 to create another type of vortex, thus also delaying the exchange of one source gas for another.
In addition, if the horizontal reactor 200 should be re-designed every time some process condition, e.g., the flow velocity or pressure of a source gas, is changed and optimized, then the productivity should decline or the costs would increase disadvantageously.
It is therefore an object of the present invention to save the need for re-designing a horizontal reactor even if some condition, like the flow velocity or pressure of a source gas, for a film deposition process to be carried in the reactor has been changed and optimized.
To achieve this object, in depositing a semiconductor film, a gas flow rate is fixed at a predetermined value according to the present invention by keeping the product of the flow velocity and pressure of source gases inside the reactor constant.
The present inventors carried out various types of research on a process for depositing a compound semiconductor film using a horizontal reactor. As a result, we found that the spatial distributions of velocity and temperature of source gases and that of the thickness of a film to be deposited on a wafer are substantially controllable in the reactor by the flow rates of the source gases. The velocity and temperature distributions of reactant gases, resulting from chemical reaction between the source gases, were also controllable by the flow rates. As is well known in the art, the flow rate of a gas is proportional to the product of the flow velocity and pressure of the gas. Accordingly, each of those spatial distributions can be kept substantially uniform during the film deposition process only if the flow velocity or pressure of the source gases is changed in such a manner as to maintain a predetermined gas flow rate.
Specifically, a first inventive film deposition method is for use to deposit a semiconductor film on a wafer by making a source gas supplied flow almost horizontally to the surface of the wafer. In this method, the source gas has its flow velocity and/or pressure changed so that the source gas is supplied at a substantially constant flow rate.
According to the first inventive method, a source gas supplied has its flow velocity near its inlet port and pressure inside a reactor changed so that the source gas is supplied onto a wafer at a substantially constant flow rate. Thus, it is clear from our findings that even if the flow velocity of the source gas is changed to deposit a film at a higher rate, the film deposited still can have its thickness uniformized. So there is no need to re-design the horizontal reactor each time the process conditions are changed.
In one embodiment of the present invention, the pressure of the source gas is preferably set within a range from about 0.01 atm and about 2 atm.
A second inventive film deposition method is also for use to deposit a semiconductor film on a wafer by making a source gas supplied flow almost horizontally to the surface of the wafer. The method includes the step of a) controlling the flow velocity and pressure of the source gas to find a first flow velocity and a first pressure that make such a combination as substantially uniformizing the thickness of the film deposited, and then determining a reference flow rate for the source gas. The reference flow rate should meet a predetermined relationship with the product of the first flow velocity and the first pressure. The method further includes the step of b) changing the first flow velocity and the first pressure into a second flow velocity and a second pressure with the reference flow rate kept constant. And the method further includes the step of c) supplying the source gas onto the wafer at the reference flow rate with the flow velocity and pressure of the source gas set equal to the second flow velocity and the second pressure, respectively, thereby depositing the film on the wafer.
According to the second inventive method, a first flow velocity and a first pressure of a source gas are changed into a second flow velocity and a second pressure with a reference flow rate kept constant. Then, the source gas is supplied onto a wafer at the reference flow rate with the flow velocity and pressure of the source gas set equal to the second flow velocity and the second pressure, respectively, to deposit a film on the wafer. Thus even if the flow velocity and pressure of the source gas have been changed, each film deposited can have its thickness uniformized. As a result, there is no need to re-design the horizontal reactor each time the process conditions are changed.
In one embodiment of the present invention, the first flow velocity is preferably determined in the step a) by setting an initial value of the first pressure to 1 atm or less. Specifically, it would be easier to find an optimum reference flow rate by setting an initial value of the first pressure to 1 atm or less and then changing the first flow velocity gradually to determine the best first flow velocity as compared to setting an initial value of the first pressure to more than 1 atm and then changing the first flow velocity gradually to determine the best first flow velocity.
Also, the first and second pressures are each preferably set within a range from about 0.01 atm and about 2 atm.
This invention also provides a method for fabricating a semiconductor device, including at least first and second semiconductor films stacked in this order on a wafer, by making at least first, second and third source gases supplied flow almost horizontally to the surface of the wafer. The method includes the step of a) controlling the flow velocity and pressure of the first source gas to find a first flow velocity and a first pressure that make such a combination as substantially uniformizing the thickness of each said film to be deposited, and then obtaining a reference flow rate for the first source gas. The reference flow rate meets a predetermined relationship with the product of the first flow velocity and the first pressure. The method further includes the step of b) setting a second flow velocity and a second pressure, which are different from the first flow velocity and the first pressure, respectively, for the second source gas with the reference flow rate kept constant. The second source gas has a viscosity substantially equal to that of the first source gas. The method further includes the step of c) supplying the second source gas onto the wafer at the reference flow rate with the flow velocity and pressure of the second source gas set equal to the second flow velocity and the second pressure, respectively, thereby depositing the first film on the wafer. The method further includes the step of d) setting a third flow velocity and a third pressure, which are different from the second flow velocity and the second pressure, respectively, for the third source gas with the reference flow rate kept constant. The third source gas has a viscosity substantially equal to that of the first source gas. The method further includes the step of e) supplying the third source gas onto the first film at the reference flow rate with the flow velocity and pressure of the third source gas set equal to the third flow velocity and the third pressure, respectively, thereby depositing the second film on the first film.
According to the present invention, first, a reference flow rate is determined for a first source gas. Next, with the reference flow rate kept constant, a second source gas is supplied at a second flow velocity and a second pressure to deposit a first film. Then, a third source gas is supplied at a third flow velocity and a third pressure to deposit a second film on the first film. Thus each of multiple films for a semiconductor device can have its thickness uniformized and its quality improved.
As used herein, the second or third source gas should xe2x80x9chave a viscosity substantially equal to that of the first source gasxe2x80x9d in the following two situations. One of the two situations is that even though the second or third source gas is made of a molecular species different from that of the first source gas, the second or third source gas has a viscosity substantially equal to that of the first source gas. In the other situation, the second or third source gas is also made of a different species from that of the first source gas and the first and second or third source gases are both diluted with a carrier gas in a huge quantity. In that case, the first and second or third source gases have their viscosities determined almost by the viscosity of the carrier gas itself.
In one embodiment of the present invention, the first and second films each preferably contain at least one Group III element and at least one Group V element. The second source gas preferably contains gallium and indium as Group III element sources and the second pressure set for the second source gas is preferably about 0.3 atm or more. The third source gas preferably contains gallium and aluminum as Group III element sources and the third pressure set for the third source gas is preferably about 1.0 atm or less. And the second pressure is preferably equal to or higher than the third pressure.
Also, the first, second and third pressures are each preferably set within a range from about 0.01 atm and about 2 atm.
The present invention further provides an apparatus for depositing a semiconductor film on a wafer by making a source gas supplied flow almost horizontally to the surface of the wafer. The apparatus includes: a reactor, in which the wafer is placed and which has a gas inlet port for supplying the source gas onto the wafer; velocity control means for controlling the flow velocity of the source gas; and pressure control means for controlling the pressure of the source gas in the reactor. In this apparatus, the velocity and pressure control means control the flow velocity and the pressure in such a manner as to keep the flow rate of the source gas near the gas inlet port substantially constant.
According to the inventive semiconductor film deposition apparatus, a film of a uniform thickness can be obtained even if the process conditions have been changed. Thus there is no need for re-designing the reactor every time the process conditions are changed.